Scripting: low level join meshes / elements (hopefully with bmesh)?
The best solution I found so far is using the bmesh.from_mesh( mesh ) method.
Apparently, if you call this method more than once, it will add the 2nd mesh to the first, thus effectively joining them:

import bpy, bmesh

bm = bmesh.new()
bm.from_mesh( mesh1 ) # Add first mesh
bm.from_mesh( mesh2 ) # Add 2nd mesh


However, this does not really help with adding individual components from another mesh or bmesh in a direct manner.